BDNF and depressive symptom measurement: Brain-derived neurotrophic factor (BDNF) plays a key role in the pathophysiology of stress-related mental disorders such as depression, and in the therapeutic mechanism of antidepressants (Nestler et al., 2002; Hashimoto et al., 2004, Duman and Monteggia, 2006; Hashimoto, 2010, 2013; Duman and Li, 2012; Lindholm and Castrén, 2014).